This invention relates to a high voltage transformer. More particularly it concerns a high voltage transformer for cascade connection where the high voltage transformer comprises a primary winding, a high voltage winding and a transformer core and wherein the primary winding and the high voltage winding encircles at least a part of the transformer core.
In the description the term “good high frequency qualities” is used. By this is meant that a so-called “pulse transformer” having relatively low coupling inductance between the primary and secondary windings, relatively low so-called “skin effect” and “proximity effect” in the windings at relatively high frequencies, relatively low parasitic capacitance internally in the windings and relatively low capacitance between windings and between windings and the transformer core. This concerns particularly the high voltage winding. Said physical parameters are well known to a person well versed in the art and are therefore not explained further.
For a pulse transformer being run near to saturation, typical for inverters, the practical expression:U=4Bs*f*n*Ae is used, where Bs=magnetic flux density (saturation), U=the top value of the voltage over the winding, f=working frequency, n=number of turns and Ae=effective cross-section of the transformer core.
From the expression appears that a high output voltage may be achieved at a high frequency, high saturation field strength, large iron cross-section and many turns.
In case of little room available it is often easiest to increase the frequency. To avoid too great eddy-current losses one then has to use core materials having low electrical conductivity such as ferrite, iron powder or so-called “tape wound cores”.
A method for feeding the transformer a relatively high frequency comprises a so-called SMPS—(Switched Mode Power Supply) technique. The input power is according to this technique converted to a preferably square pulse high frequency input voltage to the high voltage transformer.
A prior art high voltage transformer has as mentioned, due to its mode of operation, a relatively high number of turns in the secondary winding. This causes an increased secondary capacitance in that the windings with many layers of relatively thin winding wire have less mutual average distance from each other than in a transformer where the winding wire is of larger diameter.
The many turns of the secondary winding requires relatively much space and thereby leads to the transformer core and the primary winding being relatively large. In addition large insulation distances are required between high voltage winding, primary winding and transformer core. The transformer thus being relatively large leads to increased losses in transformer windings and also that high voltage transformers of this kind have a relatively low coupling factor. A low coupling factor may be modelled as a relatively large coupling inductance. The reason is that a relatively large distance between the primary and secondary windings leads to poor magnetic coupling between them.
This unintentional and in the main unavoidable parasitic coupling inductance will, in the same way as the secondary capacitance and in combination with the secondary capacitance, influence the current in the transformer. By the coupling inductance limiting the high frequent current, and also that most of this current is used to drive internal parasitic capacitance in the secondary winding, a clear limitation in the power output from the secondary winding at high frequencies arises. High frequency transformers of this kind have thus a relatively narrow bandwidth, i.e. the highest driving frequency the high frequency transformer can work at.
Known low voltage SMPS technique can produce voltages up to the order of 1 kV. At higher voltages it is necessary to adapt the transformer by means of per se known techniques as voltage multiplication, cascade coupled high frequency transformers, layered winding techniques or so-called “resonant switching” to compensate for the relatively narrow bandwidth in a high frequency transformer.
Common for all these techniques is that they only to a limited extent overcome the drawbacks at the same time as they complicate and thereby raise the price of the complete high frequency converter.
It is known to reduce the number of layers in a transformer to be able to achieve improved transformer properties. U.S. Pat. No. 7,274,281 deals with a transformer for a discharge lamp such as a fluorescent tube where the transformer is provided with two series connected primary windings that may be constituted by one winding layer.
U.S. Pat. No. 1,680,910 describes a transformer for cascade connection. This one is however not suitable for SMPS because it has a high capacitance in the windings and a low coupling factor.
U.S. Pat. No. 4,518,941 shows a transformer that is suitable for SMPS but where the rated transformer ratio is one to one. The transformer according to this document is not suitable as a high voltage transformer.
U.S. Pat. No. 3,678,429 shows a high voltage transformer for cascade coupling wherein there besides a primary winding and a secondary winding is arranged a winding for cascade coupling. Due to the design of the high voltage winding the transformer according to U.S. Pat. No. 3,678,429 is not suitable for SMPS.
U.S. Pat. No. 3,579,078 deals with a one-step transformer coupled to a so-called “Voltage Quadrupler”. The transformer does not however solve the relevant technical problem as one does not achieve a high enough voltage in one step.
From WO 2007045275 it is known to use two secondary windings for cascade coupling with a so-called “flyback-convertor” to achieve a stable output voltage in each cascade step.
Prior art does not exhibit transformers having suitable high voltage properties and at the same time being suitable for cascade coupling.